When choosing vitamin D supplements, it’s important to look for products with independent quality control testing, such as a USP seal – this ensures nutrient and contaminant testing but still does not guarantee exact nutrient content.

We know that vitamin D is an important hormone that helps our digestive systems absorb calcium and phosphorus – and that it’s a necessary nutrient for human health.

Dr. JoAnn Manson, P.H. is the chief of the division of preventive medicine at Brigham and Women’s Hospital and professor of medicine and the Michael and Lee Bell professor of women’s health at Harvard Medical School. She is also a study director of VITAL, the single largest ongoing research study of its kind on the effects of vitamin D and omega-3 supplements across over 25,000 participants.

The results of this study were published in the November 2018 issue of the New England Journal of Medicine. It’s a significant study – Clifford J. Rosen of the Maine Medical Center Research Institute in Scarborough told Vox that “This is the largest and most definitive [vitamin D] trial with long enough follow-up and substantial representation from minorities to be an excellent cross-section of Americans.”

INSIDER spoke with Dr. Manson who shared her expertise on the subject. Here’s what you need to know about getting the correct amount of vitamin D from sources other than the sun.

First, it’s important to understand which claims about vitamin D have been scientifically proven – and which haven’t

“There are a lot of observational studies which are not randomised clinical trials suggesting that a low vitamin D level is associated with an increased risk of many chronic diseases. We say in epidemiology that correlation does not prove causation,” said Dr. Manson.

She continued, “It really requires a randomised clinical trial to prove a cause-and-effect relationship that vitamin D supplements will lower the risk of these chronic diseases. So far, the randomised clinical trials have suggested limited benefits beyond bone health.”

“VITAL does suggest a promising signal for cancer death, but it’s not conclusive. [VITAL results showed that] there was no reduction in cardiovascular events or total cancer incidence,” she added.

The current recommended dietary allowance of vitamin D was created with people with no sun exposure in mind

Dr. Manson told INSIDER, “There’s a lot of controversy on the subject, but the Institute of Medicine has determined that the recommended dietary allowance (RDA) for vitamin D is 600 to 800 IUs a day for the general population.

“That would meet the needs of 97.5% of the general, healthy population. Of course, people who have osteoporosis or bone health problems or a malabsorption condition, or who are taking certain medications that interfere with vitamin D metabolism may need a larger amount than that.

“So clinicians have a lot of latitude in deciding the amount of vitamin D that people need. However, the 600 to 800 IUs a day was determined on the basis of minimal to no sun exposure – you know, what people would need during the winter, at high northern latitudes.

“So even people who are living in different parts of the country – these guidelines are for the US and Canada – 600 to 800 IUs a day should cover their needs.”

There are dietary sources of vitamin D – but they’re somewhat limited

ShutterstockFish is a great source of vitamin D.

“If you’re eating a balanced diet – people who don’t get any sun exposure – it should be perfectly fine to take a supplement of 1000 to 2000 IUs a day. However, many people can get the vitamin D that they need if they get the major dietary sources of vitamin D, such as dark fatty fish, fortified dairy products, fortified cereals, and some people will take a multivitamin that will give them a little extra, so that is often enough,” Dr. Manson told INSIDER.

If you find that you are not getting enough vitamin D through other means, supplements can be both useful and safe

PexelsVitamin D supplements have found to be safe but still talk to your doctor before taking any supplement.

Dr. Manson told INSIDER, “Certainly, taking a supplement is reasonable – we showed in the vitamin D/ omega 3 trial that taking 2000 IUs a day was safe – over 5.3 years, there was no increased risk of high blood calcium or gastrointestinal problems, so that was shown to be safe.

However, dietary sources of vitamin D are somewhat limited and some people may not get enough from diet. Especially above age 70, that’s where 800 IUs a day is recommended, and that may be difficult to achieve from diet. So taking a supplement of 1000 to 2000 IUs a day is very reasonable if you have any concerns about whether you’re getting enough from diet.

If you have a special requirement due to medical issues, then you may require even more than that.”

Vitamins are classed as dietary supplements in the US – and regulation is minimal, but there are ways to be reasonably sure of what you’re getting

“You need to be careful if you’re taking over-the-counter supplements to look on the label for some evidence of quality control testing. This would include looking for the US Pharmacopeia (USP) seal, looking for something like NSF or ConsumerLab.com testing. You need to look for evidence of some independent quality control testing.

Very often, it’s in the form of the USP seal. But it doesn’t guarantee that the nutrient content is exactly what’s stated on the label. It is evidence that it’s been tested for various things, like nutrient content and contaminations.”

We need a certain amount of vitamin D for good health – but it’s possible to get too much, which can present a separate set of problems

Dr. Manson told INSIDER, “At this point, we have shown that taking 2000 IUs a day is safe – there isn’t a high blood calcium level that develops. It seems to be safe in terms of no real side effects or any clear risk. Certainly, if your doctor or healthcare provider has suggested that you take vitamin D, the VITAL trial would not suggest reasons to stop.

If you’ve been taking it at similar doses on your own, just over the counter, there’s no clear reason to stop. But avoid mega-dosing. The very high doses – especially once you get to 10,000 IUs a day or higher – have been linked to risk, and there isn’t evidence that those high megadoses are safe when taken long-term. So we do want to caution against megadoses.”

Megadosing can result in vitamin D toxicity – with side effects including elevated blood calcium levels, nausea, vomiting, gastrointestinal distress, bone loss, and even kidney failure according to Healthline.

The VITAL study is ongoing – so there’s likely more to learn from this research

ShutterstockStudying the effects of vitamin D might help treat conditions like depression in the future.

Although initial results have recently been published, Dr. Manson told INSIDER that participants will keep going for several more years.

Dr. Manson told INSIDER that researchers will follow participants “at least to two years after the intervention and the treatment phase, and then we’re going to try to follow them for an additional four to five years after that.

“So we hope to be able to follow them for six to seven years after stopping treatment to see if the reduction in cancer risk or cancer death becomes stronger over time.

“We’ll also have additional research coming out from VITAL on cognitive function, diabetes, depression, and autoimmune diseases. These findings could help to inform decision-making.”